Dean’s Medal, best Honours degree in the Faculty of Natural and Agricultural Sciences, University of the Free State, 2007

PPS Prize, best B.Sc. Honours student, University of the Free State, 2007

Botanical Society of South Africa Award, best honours student in Botany, University of the Free State, 2007

CONFERENCE PAPERS PRESENTED

De Waal, C., Venter, A.M. & Miller, J.S. Breeding systems of two South African Lycium species (Solanaceae). Annual conference of the South African Association of Botanists. January 2008, Drakensville, South Africa.

EXTRACURRICULAR ACADEMIC EXPERIENCE

Graduate Teaching Assistant (2008), first year biology labs, University of Toronto

Graduate Teaching Assistant (2007), second and third year botany labs, third year botany field excursion, third year mycology labs, University of the Free State, South Africa

Teaching Assistant (2006-2007), first year biology labs, University of the Free State, South Africa

SUMMARY OF B.SC. HONS. THESIS

The cosmopolitan genus Lycium (Solanaceae) consists of ca. 80 species. Lycium species vary in sexual function, with many species being hermaphroditic, and some species either gynodioecious (Miller & Venable, 2002, 2003) or dioecious (Venter, 2000, 2007). The genus is an interesting system for the study of the evolution of sexual dimorphism, since atypically it has occurred in the presence of self-incompatibility (Miller & Venable, 2002). Miller and Venable (2000, 2002) addressed this relatively uncommon association by proposing that separate sexes evolved in Lycium following polyploidy which caused the breakdown of self-incompatibility, inbreeding depression and the conditions favouring the evolution of gender dimorphism.
Recent phylogenetic studies of the genus (Levin & Miller, 2005; Levin et al., 2007) provide support for a South American origin of the genus. Self-incompatibility is ancestral in the genus and phylogenetic studies suggest a single dispersal event for Lycium from the New to the Old World, since Old World Lycium species are monophyletic (Levin & Miller, 2005; Levin et al., 2007). However, Baker’s Law (Baker, 1955, 1967) states that a new coloniser would more likely be self-compatible than self-incompatible. We therefore expect to find that the establishment of the genus in Southern Africa was facilitated by a loss of self-incompatibility in the first colonists. Self-incompatibility would then be expected to be absent in Southern African Lyium, since a complex trait such as self-incompatibility can be lost relatively easily, but its loss is essentially irreversible (Igic et al., 2003).
To determine the compatibility status of species in the monophyletic African Lycium, I studied the summer-flowering L. cinereum Thunb. and the winter-flowering L. hirsutum Dunal, representing two different clades of the genus. I compared fruit and seed set following controlled cross- and self-pollination through field investigations and determined the presence or absence of self-incompatibility. I also investigated pollen-tube growth in L. hirsutum using fluorescence microscopy.
My results demonstrate that both L. cinereum and L. hirsutum are self-incompatible, based on fruit and seed production in controlled crosses and observations of pollen-tube growth. This in one of the first reports of self-incompatibility in Southern African Lycium species. Self-incompatibility has also recently been reported in L. ferocissimum and L. pumilum (Miller et al., 2008). Collectively these results indicate that self-incompatibility was maintained in African Lycium after long-distance dispersal from the New World and provides an interesting exception to Baker’s Rule (1955, 1967).

I am interested in the mechanisms governing sex ratios in dioecious flowering plants, particularly those related to female-biased sex ratios. Sex ratios often deviate from 1:1, with male bias being more common than female bias (Delph, 1999). Various mechanisms can influence sex ratios, and to understand the evolution of sex ratios we need to determine when and how biases are established during the life cycle of the plant (Lloyd & Webb, 1977; Bierzychudek & Eckhart, 1988; Ågren et al., 1999; Taylor, 1999; De Jong & Klinkhamer, 2002; Taylor & Ingvarsson, 2003).

Some of the main hypotheses that have been proposed to account for female-biased sex ratios include 1) certation (Correns, 1922) – the selective fertilization that result from differential pollen-tube growth of female- versus male-determining microgametophytes; 2) gender-specific mortality, i.e. differences in the performance and viability of the sexes after parental investment (Zarzycki & Rychlewski, 1972), and 3) sex ratio distorters (Taylor, 1999).

Female-biased sex ratios, although infrequent, are often reported in species with heteromorphic sex chromosomes (Lloyd, 1974). This suggests a relation between the sex determination system and either the performance of the sexes, or the performance of the sex-chromosomal genotype of gametophytes.

Sex ratios in Rumex

Rumex (Polygonaceae) is a useful genus to study mechanisms governing sex ratios, because some of the wind-pollinated dioecious species in the genus exhibit female-biased sex ratios. These species are also characterized by two distinct sex-determining mechanisms and sex-chromosome systems. In some Rumex species sex is determined by an XX/XY chromosome system, with XX being the female genotype and XY the male genotype (Löve, 1944; Smith, 1969). In other species, such as R. nivalis, a more complicated XX/XY1Y2 sex-chromosome system occurs (e.g. Smith, 1969). R. hastatulus is unique in being polymorphic for its sex-determining system with both an XX/XY and an XX/XY1Y2 sex-chromosome system.

R. hastatulus is a wind-pollinated, dioecious annual herb that occurs on coastal plains and disturbed areas in sandy soils throughout south-eastern United States (Figure 1). Populations occurring from North Carolina to Florida and Mississippi (the North Carolina race) are characterized by the XX/XY1Y2 sex-determining system (2n = 8 in females and 2n = 9 in males), whereas populations that occur from Louisiana to Texas and Oklahoma (the Texas race) possess the XX/XY sex-determining system (2n = 10 in both sexes). A third chromosome race, termed the southern Illinois-Missouri race, is mentioned by Smith (1969).

As part of my PhD project, I aim to explore the following topics pertaining to sex ratios in Rumex hastatulus: 1) Sex ratio variation between the different chromosomes races, and the possible association between sex ratio and population size and/or density; 2) The sex ratios of open-pollinated seed arrays, and the effect of the maternal mating environment; 3) Mechanisms that could result in biased sex ratios, such as certation, sex-specific mortality, and sex ratio distorters.